Tube mic PSU with voltage tripler - Transformer power rating?

[pasarski]

Hello,

I 'm building a PSU for a tube mic based on a Royer mod PSU. Can't find the schematic published online and don't want to make a copyright violation but it's based on a voltage tripler, 105V B+ tripled from a 24V transformer, ~6V for the heater tapped from a approriate point of the tripler circuit.

What I'm wondering why and how the voltage tripler circuit wastes so much current that the power transformer must be rated 24VA like in the Royer schematic?

5840 heater draws 150mA.  I'm using 5670 tube in the mic with 6.3V 350mA heater. Do I really need a 56VA transformer or could it work with little less, like 40VA (there's one that would fit my casing in the local store). My hunch is that it will, but can't calculate it to be sure and don't want to waste money for the transformer if it won't work.

P.S.  It's even recommended to use 50VA transformer here by Gus:

http://www.groupdiy.com/index.php?topic=31477.0

Did you mean you use this kind of supply with higher heater current tubes and thats why the even higher VA?

 

[soapfoot]

I made a pair of Royer mods with a dual PSU that has two voltage tripler PSU boards running off of one 40VA transformer.  No problems yet in about 3 years of regular use.  I also have a light bulb on the secondary, too, that draws a bit of current.

 

[PRR]

> ~6V for the heater tapped from a approriate point of the tripler

There's no "approriate point" in the tripler. I assume it taps a 35V node and wastes 5/6th of that to get 6V. Usig round numbers you may want 8VA for heat, the plate power is negligible, 10VA may be ample.

 

[pasarski]

Usig round numbers you may want 8VA for heat, the plate power is negligible, 10VA may be ample.

Round numbers like 10V X 0.4A = 4VA and because voltage is tapped  from the "double voltage point" of the tripler 8VA transformer is needed?

BTW I tried the PSU with a 8VA transformer I have and it didn't work, B+ and heater voltage dropped more than they should have.

Anyways, seems like this is going to be a mad power waster. I actually want a higher B+ and was going to use 30V transformer, that makes the amount of the voltage drop needed even crazier. Maybe I should just try to find a separate 4VA transformer for the heater that I can fit in to the box?